Having written recently about the U-Boat U-1001/1, I took a brief plunge into the world of diving watches and what they offer. There are very specific requirements for a watch to be considered a diving watch. While these features are outlined in detail by international standards, the most important ones can be summarized by considering what is essential to underwater timepieces. They must function when submerged and under great pressure and allow the wearer to easily and quickly measure the elapsed time at different crucial instances during the dive.

The more one knows about scuba diving, the more it becomes clear that the use of mechanical watches for diving purposes is a simplistic and obsolete method, and that they should probably only be used for recreational use at relatively shallow depths. We are mostly interested with luxury watches, so this article will focus on what makes these watches capable of accompanying divers to some of the deepest parts of the earth and how this translates to the quality and allure of these tough and tantalizing timepieces during everyday use.

Water Resistance

Not surprisingly, the first preoccupation of a diving watch is to keep water out. Impermeability is measured by way of different water resistance ratings. Different international standards describe water resistance depending on the purpose of the watch that is being tested. For watches that are not considered fit for diving an exaggerated rating is issued. A 30 meters inscription on a watch dial tells me that one should abstain from looking at water with this piece on their wrist. The explanation for this seemingly deceptive methodology is that the watches are tested in a static pressure setting and don't account for the watch moving inside the water. Underwater motion of the arm for instance, can add considerable pressure to the watches surface. A different logic is adopted when it comes to water resistance test for diving watches. The watch is tested at 125% of its desired water resistance rating, therefore taking into consideration the motion of the diver's arm under water. Furthermore, for this type of advanced rating, each individual watch that is manufactured must be tested.

What makes a watch so resilient to water pressure? The most simple and obvious elements play an important role. Rubber gaskets line all the components of the case that can leak. The crystal, case-back, and crown are all sealed by one or more gaskets each. A screw-locking crown is indispensable to elevated water resistance. It acts like a cap sealing a bottle or canteen. The effectiveness of screw thread to create a seal is also the reason why all water resistance watches have a case back that screws into the case. Chronograph pushers, which unless specially built should never be used underwater, are often screw-locking as well to avoid accidental deployment. The Bell & Ross BR-02 94 exemplifies this well, as it is equipped with two screw-down crowns and screw-down pushers. It is rated to be water resistant to 500 meters.

At greater depths, the pressure not only threatens to push the water in through the seals, but also has the ability to crack, or even shatter the crystal. This is why the massive chucks of synthetic sapphire are applied to submersible timepieces. The U-Boat U-1001 is equipped with a crystal that is 5.5mm, or roughly one quarter of an inch thick. The best example of a thick crystal has to be the 1960 Deep Sea Special which accompanied the infamous Trieste submersible vessel to the bottom of Challenger Deep, over 11,000 meters under the surface of the Pacific. Its acrylic crystal looks like a deformed, transparent golf ball.

In the same spirit as Rolex's Deep Sea Special, other watchmakers are inclined to innovate and push the limits of what pressure their watches can withstand. A relatively recent method involves filling the entire watch with mineral oil. Bell & Ross adopted this idea for their , which can withstand pressures of up to 1,110 bars, equivalent to what is felt at 11,100 meters depth.

An element commonly found on diving watches is the mysterious helium escapement valve. At least it was mysterious to me until I looked a little further into it. When I first started to work with high-end watches, I imagined that there was helium inside the watches quipped with this feature. Wrong! I then thought that this valve was meant to compensate for the deformation of the watch case due to extreme pressure. I understand now that the escapement valve is more important when coming up from a dive then going down. Furthermore, only saturation diving benefits from a helium escapement valve. Saturation diving is useful for reaching important depths greater than 50 meters. The diver is placed in a chamber, often a diving bell, filled with a gas mixture that contains helium. This acclimatizes divers to whatever pressure they will be working at as well as reducing the physiological effects of scuba. Because helium is a very small molecule it seeps past the gaskets of the watch and waits. When resurfacing the decrease in pressure causes the Helium to expand and without the valve, would damage or pop the crystal directly off.

Legibility and Functions

Structurally a diving watch has to keep water from seeping in and not collapse under great pressure. In order to be useful there are certain basic functions that it must incorporate. Probably the most important ability is to be able to tell the wearer how long he or she has been underwater and therefore determine how much air is left in the tanks. This is why all true diving watches are equipped with some sort of rotating bezel. The bezel, which must be graduated with intervals at least as precise as five minutes, is used in conjunction with the minute hand to mark the beginning of a dive. This also explains why a rotating bezel on a diving watch must be unidirectional. Any accidental sliding of the bezel will be counter-clock wise, so it will appear as if you have less time than you actually do, a much less dangerous situation than if you thought you had more air in your tanks than there actually is! Other methods are occasionally employed to secure the bezel in place like is seen on the U-Boat U-42 which uses a lever to locks the bezel solidly in its position.

The bezel rotates freely when the specialized lever of the U-Boat U-42 is pulled and locks into place once the lever is tucked back into the case.

Once underwater, it can get dark pretty quickly and this explains why diving watches are always adorned with very luminescent markings on dial and bezel. The Alpina Avalanche Extreme Diver, for example, doubles for a flashlight when the lights are out. Most of today's watches use phosphorescent materials, which simply put, absorb light and re-emit it slowly even in the dark. Seeing how much light modern watches emit, this approach strikes seems safer than the original method of using radioactive Tritium to produce luminescence.

Diving Requirements Translate to Everyday Wear

The truth of the matter is that mechanical watches are oversimplified, rudimentary tools for calculating dive and resurfacing times. They are only really practical for light, recreational diving at minimum depth. While the tough, rugged structure of the watches will withstand pressures at amazing depths, it is the functions that are lacking. Dive computers are able to calculate depth, water temperature, perform continuous calculations, warn of missed decompression stops, and more. While some mechanical watches are able to calculate depth and others are able to run a chronograph underwater, these are rare and still lag behind basic dive computers for functionality.

What they lack in functionality, diving watches make up in appeal. All the requirements that a timepiece must meet to be considered diver's translate perfectly into desirable features for a luxury sport watch. A watch equipped with robust diving features is necessarily a tough piece. A thick crystal protects it in case of being dropped for instance. The crown is the most delicate part of any watch, so screw-locking crowns and pushers required for deep water submersion are welcome reinforcements.

Diving watches display a specific design, which over the years has become one of the most sought after looks. The popularity of the Rolex Submariner is proof. Graduated bezels and a rugged sporty cases are what make up some of our favourite timepieces. Books, Websites, and collectors clubs have been dedicated to this field of horology and are not exclusive to practising divers, but patronized by pure, watch enthusiasts as well.